Studies of trace elements in the Sargasso Sea and the Mid Atlantic Bight

Jickells, Timothy David (1986) Studies of trace elements in the Sargasso Sea and the Mid Atlantic Bight. University of Southampton, Doctoral Thesis.

Abstract

Methods have been developed for the measurement of dissolved A1 and total dossolvable Cd, Cu, Co, I, Mn, and Ni in ocean waters. These methods have been applied to samples collected from the Sargasso Sea and the Mid Atlantic Bight on the North American continental shelf. In all cases dissolved species dominate. In the Sargasso Sea, the distributions of Cd, Ni and I are controlled by nutrient-like cycling, though the carrier phases involved and the scale of the cycling vary for each element. For I, biological cycling also reduces 10₃- to I^-, which is then only slowly oxidised. The distributions of A1, Mn and Co are dominated by surface (probably aeolian) inputs with subsequent scavenging throughout the water column, though the rate of this varies from element to element. In deep water, changes in concentration in association with different water masses are evident. The distribution of Cu is controlled by biological uptake and regeneration coupled to scavenging throughout the water column with extensive regeneration from sediments. In shelf waters, the distributions of Cd, Cu, Mn and Ni appear to be dominated by conservative mixing of oceanic waters of low concentration with riverine waters of higher concentrations. Intense scavenging produces very low A1 concentrations in shelf waters, which then appear to mix conservatively with oceanic waters of higher concentration. The Gulf Stream restricts direct transport of shelf waters to the central ocean, except within Gulf Stream eddies. In a time-series of measurements in the surface waters of the Sargasso Sea, Cu, I and Ni showed little seasonal variability, though the speciation of I varies with maximum concentrations of I^- in summer, when seasonal water column stratification isolates the surface waters. This stratification also leads to increases in A1 and Mn concentrations above the seasonal thermocline, due to the confinement of atmospheric inputs to the shallow mixed layer. Particulate elements associated with clay particles (A1, V and Ti) also show a maximum in concentration above the seasonal thermocline. By contrast, particulate Ca, Mg and I are associated with biogenic particles, which dominate the total suspended matter throughout most of the year, and show concentration maxima during the spring phytoplankton bloom. Particulate Ba and Mn appear to be formed authigenically within the surface waters. Comparisons of atmospheric inputs, primary production rates and observed changes in concentrations of trace elements in surface waters suggest a complex inter-relationship between these factors. Comparisons of atmospheric inputs with deep ocean sedimentation rates measured with sediment traps, confirm the importance of atmospheric inputs for Cd, Cu, Ni, Pb and Zn, while for A1, Mn and Fe lateral transport of particulates in the deep ocean is a major source to the sediments of the Sargasso Sea. While the Sargasso Sea is a net sink for A1, Mn and Fe, a large proportion of the Cd, Ni and Zn, and some of the Cu, introduced to the Sargasso Sea is exported southward in the North Atlantic Deep Water. (D73652/87)

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